Preparation of the Wall Surface
The successful application, bonding and correct hardening of
hydraulic lime mortars, requires that the background should be
clean, free from vegetation, free of containments and reasonably dry
throughout the wall mass. The wall should be structurally sound and
the masonry and bedding mortars in good condition.
The application of various dubbing out coats should not be seen as
remedial repair work to the masonry or brickwork background, the
replacement of missing or damaged masonry or open joints should be
repaired before any plastering of dubbing out coats are applied.
Where excessive hollow or unevenness is present the areas should be
corrected by use of mortar and stone or brick slips bedded into the
mortar, with the aim of presenting a reasonable flat background
before plastering commences.
Suction Control and Bonding
Before the application of any new lime coatings, hydraulic or
non-hydraulic, it is vitally important to check to the degree of
suction within the background, poor or excessive suction can result
in a weak bonding with the substrate caused by rapid de-maturing of
the newly applied render, which will result and a weak and powdery
interface which will lead to later failure and separation. Where
there is little or no suction further action will be required to
help bond the coating to the substrate. In situations where suction
needs to be controlled, wetting down will be required, on dense
blocks or near impervious masonry, simply dampening the surface with
a mist spray may be all that is required, but on very porous
surfaces such as old brickwork considerable wetting will be
required. Wetting the wall by use of a hose, working from the top of
the structure, downwards, may need to be carried out the previous
day or several times throughout the day before rendering commences.
The objective of the suction control is to achieve a thoroughly damp
surface, but not wet, i.e., the surface must not have running or
standing water remaining on the masonry or brick, this will form a
barrier between the coating and substrate, also lime mortars adhere
and stiffen through a certain amount of suction.
On dense or near impervious background, it may be necessary to apply
a sand/splatterdash coat to the background to act as a mechanical
key.
Salt Contamination
Where new lime coatings are to be applied to masonry which is salt
contaminated, the masonry should be allowed to dry fully before
applying new renders. This will allow salt to be detected on the
masonry and mortar joint surfaces, if excessive salt is identified
clay or lime mortar poulticing may be required. Specialist advice
should be sought, if the technique is considered where salt is
detected on the mortar joints, rake out the joints to a depth of
50mm, as this is likely to be heavily contaminated and in a weakened
condition and repoint (See repointing).
Salt contaminates should never be washed from the surface, as this
will result in the crystallized salt returning to a soluble state
and retreating back into the pores of the masonry or brick. Where
detected on the masonry surface, the salts should be brushed from
the surface and cleaned away from the structure.
Techniques of Hydraulic Lime Plastering
The techniques employed in the application of hydraulic lime
plasters should be to ensure a correct bonding with the background
while striving to minimise shrinkage and rapid drying, these
techniques should be followed throughout the plastering process.
Lime plastering is generally applied in 3 coats, but it is common to
find 2 coats or even single coat work in vernacular or early
structures. In 3 coat work the first coat on masonry or brickwork is
generally known as the scratch coat or render coat, this coat is
applied in a coat of approximately 10mm thickness, this coat can be
applied by use of a steel trowel or thrown onto the wall by use of a
harling trowel and then flattened in by the steel trowel. When this
coat has firmed up but has not gone hard, the plaster is keyed or
scratched up to produce a key for the following coats. The keying up
is carried out by use of a lath scratcher or similar toothed
instrument and care should be taken not to cut through the plaster
coat back to the background.The keying up is generally in the
shape of diamonds of approximately 30mm spacing. This coat should be
allowed to harden for 72 hours minimum before further coat of
plaster are applied, before applying the second coat the first coat
should be checked for shrinkage cracks, and these should be filled
with plaster before proceeding with further coats, before the second
coat is applied the first coat should be brushed down to remove any
dust which may have blown onto the surface. The first coat should
then be damped down to ensure that the second coat is applied to a
damp but not wet surface. This second coat is called the floating
coat and is the coat which is straightened to ensure a flat and even
surface, after this coat has been straightened, the surface of this
coat is scoured up with a timber or polyurethane float (rubbing up
with a float similar to finning up render).
The purpose of the scouring is to compact the plaster and counteract
shrinkage. This process is generally required to be carried out at
least twice, during the scouring up, any small holes should be
filled before the finishing coat is applied. This coat should also
be allowed to dry and harden for 72 hours before applying the final
coat.
After once again damping down the floating coat, the final coat is
applied. This coat known as the setting coat, is applied in two
layers, coat on coat, with the trowel. This coat is applied in a way
similar to skimming. This material should be of a wet enough
consistency to allow a long and even spread. Once the setting has
been applied, it is scoured in a similar manor as the floating, to
counter-act shrinkage and then the surface can be left with a rubbed
up rendered type finish or polished smooth with a steel trowel.
General Mix Guide
Backing Coats 1st and 2nd
1 Part NHL2 or NHL3.5 to 2Parts or 2.5Parts Washed Sharp Sand
Use of fibre or hair in these coats is recommended
Finish Coat
1 Part NHL2
1 Part Fine Silica Sand
Note
Highly polished surfaces will not allow the passage of moisture as
well as an open textured surface.
 |
Sand Selection |
Mortar Mixing | Pointing
| External Rendering |
| Internal Plastering |
Harling/Roughcasting | Limewashing
| Limecrete | Aftercare
|
|
Roundtower Technical Helpline
